We'll never know which film might have been the 10th best picture nominee at this year's Oscars.

Ahead of Hollywood's biggest night on Sunday, moviegoers may wonder why there have been only nine nominees for best picture the last three years — even though the Academy of Motion Picture Arts and Sciences' rules allow for up to 10.

It's all part of a magic, complicated formula intended to choose the year's best films, introduce an element of surprise, and minimize voters' incentive to vote strategically, according to Rick Rosas, a PricewaterhouseCoopers partner who supervises the count.

It allows for fewer nominees in years when support is weak for certain films. Movies that make the cut need a hard-core group of supporters as well as broad acceptance by voters who would rank it their second- or third-best choice of the year.

“We want voters to be able to fully vote their conscience,” Rosas says, “not to worry, 'My vote won't matter for a particular film.'”

Here's how the best-picture nomination process works. But we must also issue a spoiler alert: Stop reading now if you would rather not spoil the fun of the Oscars by trying to figure out one of the most complicated formulas devised by man.

Still with us? Good luck.

The Academy's 6,000 members are sent ballots and asked to rank their five favorite films of the year.

All the films are ranked according to how many first-place votes they received.

Any that get over 9.1 percent of the first-place votes are automatically nominated. That ensures that no more than 10 nominees automatically make the cut. If 10 make it, then the counting is done.

If there are fewer than 10, the ballot counters look for so-called surplus votes. Any film that receives votes at least 10 percent above the 9.1-percent automatic nomination threshold will have the surplus votes re-allocated according to the voter's second choice — or third choice if the second choice is already nominated.

Each first-place vote for that popular film is given a lesser weight so the film barely crosses the threshold, and that ballot's second-place votes help another film, to a lesser degree.

So if “12 Years a Slave” received 10 percent of the votes, all of its first-place votes are counted with a weight of roughly 0.9 votes, and all of the second-place votes on the same ballot are redistributed to those films (like “American Hustle”) with a weight of roughly 0.1 votes.

Another check is done to see if any films cross the 9.1 percent threshold.

Next, films with less than 1 percent of the first-place votes are eliminated and those ballots are reallocated according to the voter's second choice — or third choice if the second is already nominated.

Finally, all films with more than 5 percent of the votes are automatically nominated. If there are between five and 10, the counting is done. If there are more than 10, the films with the fewest votes are eliminated and their next-choice votes are redistributed until only 10 remain. If there are less than five, the counters start over and eliminate from the least vote-getters up, while redistributing voters' next-choice votes until only five remain.

So those second- and third-place choices really matter, even if a voter's first-place selection is extremely popular.

Still with us? The actual balloting for best picture — completed this past Tuesday — is far less complicated:

All voters are again asked to rank the nominees in order of preference — this year, that means from 1 to 9.

If any film gets 50 percent of the first-place votes plus one, it wins. If not, the films with the fewest first-place are eliminated and those ballots are redistributed according to the voter's second-, third- and fourth-place choices accordingly.

Either way, a film needs a lot of first-place votes from passionate backers, and a healthy standing as a second- and third-place pick in order to win.

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